Various portable electronic measuring instruments are available. One such instrument is a digital ruler with a sliding position indicator. One example of a digital ruler is described in U.S. Pat. No. 4,974,164, which is directed to a ruler-like measuring and calculating instrument that is capable of registering individual size measurements and displaying numeric results. A slider-mounted pointer is attached to a slider assembly that is received in a track in the upper surface of the device. The slider assembly includes a display and a set of control push-buttons which implement various measuring functions. The pointer points to a ruled scale along the edge, and as the pointer is moved along the scale, the position is sensed and encoded electro-optically from an internal fixed bar-scale and is then provided on the display.
Another digital ruler device is described in U.S. Pat. No. 4,282,571, which discloses an electronic apparatus for measuring, computing, and displaying linear or angular displacements. A slidable cursor selector is received, and moves in, a slot in an upper surface of the device, and points toward a straight edge on the side of the device. A display, keypad and switches for selection of various functions and operating modes are provided. In a measuring mode, an illuminated indicator representing an index is placed at one limit of the line or angle to be measured, and the slidable cursor selector is moved to the other limit of the line or angle. The apparatus then displays the associated angle or distance.
Another digital ruler type device is described in U.S. Pat. No. 4,941,267, which is directed to a method and apparatus for obtaining a distance between points which appear on a scaled drawing. First and second indicators are received in and slide along an elongated tubular opening in the side of the housing of the device to mark the points to be measured. A display and switches are provided for various functions. The indicators are pointed and extend away from the housing to assist with the location of the first and second points on the drawing. The first and second indicators are connected to a conductive arm and a resistance arm, respectively, to provide a potentiometer for generating an analog voltage signal, which corresponds to the distance between the two points. A scaled measurement is provided on the display.
Another measurement device is described in U.S. Patent Pub. No. 2004/0080306, which discloses an apparatus and method for measuring angular or linear displacement. A movable conductive member is displaced to a position at which it electrically contacts a corresponding one of a plurality of fixed contacts, thereby completing a circuit that includes a corresponding memory location in a read-only memory. That memory location indicates the amount of displacement that corresponds to that fixed contact. The corresponding data is transferred to an output device for display or additional processing.
Sliding type variable resistance devices such as those disclosed in U.S. Pat. Nos. 4,101,864; 3,671,915; 5,673,015, and the like, may also have certain features in common with some digital ruler devices. However, the features of sliding type variable resistance devices are generally not compatible with the ergonomic characteristics required for digital rulers.
It would be desirable to provide improved characteristics for a digital ruler, in comparison to the various devices referred to above. For example, improving the level of measurement accuracy while reducing the cost and/or improving the ergonomic characteristics of digital rulers, and especially the frictional characteristics, would be desirable. In particular, previously known devices have not adequately considered problems related to operating such devices using the fragile tip of a pencil, or the like. The present invention is directed to a digital ruler with an improved combination of ergonomic characteristics, low cost, reliability and accuracy.